Module 10A - Heat Exchangers I

ME 524 Heat Exchangers I

Page 1

  • Course title: ME 524 – Air Conditioning

  • Institution: Kuwait University College of Engineering and Petroleum

  • Instructor: Dr. Ammar M. Bahman

Page 2

Lecture Outline

  • Types of Heat Exchangers

  • Overall Heat Transfer Coefficient

  • Fouling Factor

  • Analysis of Heat Exchangers

  • Log Mean Temperature Difference (LMTD) Method

  • Counter Flow Heat Exchangers

  • Multipass and Cross-Flow Heat Exchangers: Use of Correction Factor

  • Effectiveness NTU Method

  • Heat Exchanger Selection

Page 3

Definition of Heat Exchangers

  • A heat exchanger is a device that transfers thermal energy (enthalpy) between:

    • Two or more fluids

    • A solid surface and a fluid

    • Solid particulates and a fluid

  • These exchanges occur at different temperatures and in thermal contact.

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Functions of Heat Exchangers

  • Heating

  • Cooling

  • Evaporation

Page 5

Classification of Heat Exchangers

  • By Direction of Flow:

    • Counter Flow

    • Parallel Flow

    • Cross Flow

    • Hybrid Flow

Page 6

Classification by Fluid Type

  • Gas-to-Gas

  • Gas-to-Liquid (evaporator, condenser)

  • Liquid-to-Liquid

Page 7

Classifications Based on Flow Patterns

  • Single Pass

  • Multi Pass

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Classification by Shape & Geometry

  • Shell & Tube

  • Double Pipe

  • Plate Type

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Heat Exchanger Selection Criteria

  • Pressure limits (high/low)

  • Thermal performance

  • Temperature ranges

  • Pressure drops across the exchanger

  • Fluid flow capacity

  • Cost

  • Cleanability, maintenance, and repair

  • Construction materials

Page 10

Working of Tubular Type Exchanger

  • Components: Tube, Shell, Baffles, Tube Sheet, Inlet/Outlet for both Shell and Tube

Page 11

Shell and Tube Heat Exchanger

  • Most common type used in:

    • Oil refineries

    • Large chemical process plants

  • Consists of:

    • A shell (large vessel) with a bundle of tubes inside

Page 12

Types of Shell and Tube Heat Exchanger

  • U-Tube Heat Exchanger

    • Shell-side fluid flows around a tube sheet and baffles

  • Straight-Tube Heat Exchanger

    • One Pass

  • Tube-side configuration with multiple passes

Page 15

Tube Layout Patterns

  • Triangular:

    • Accommodates more tubes

    • Generates high turbulence

    • Suitable for clean shell side services

  • Square:

    • Required where cleaning is essential

    • Produces low turbulence and accommodates fewer tubes

Page 16

Components of Heat Exchangers

  • Channel cover

  • Stationary head channel

  • Channel flange

  • Pass partition plate

  • Tube sheet

  • Shell flange

  • Tube

  • Shell components

  • Baffles

  • Floating head backing device

  • Floating tube sheet

  • Floating head

  • Floating head flange

  • Stationary head bonnet

  • Heat exchanger support

  • Shell expansion joint

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Tube Sheets

  • Typically 100 mm thick

  • Forged discs and clad plates for high integrity services

  • Tube-to-tube sheet joints via:

    • Expansion of tube ends

    • Explosive expansion

    • Welding

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Double Pipe Heat Exchanger

  • Concentric tube heat exchanger

  • Fluid to be cooled/heated passes through one tube, while another fluid passes through the outer tube.

  • Advantages: Low design and maintenance costs

  • Disadvantages: Low efficiency and requires more space.

Page 20

Air Cooled Heat Exchanger (AFC)

  • Device for rejecting heat from fluid to ambient air

  • Allows placement away from water supply.

Page 21

Advantages of Air Cooled Heat Exchanger

  • No piping system for air is required

  • Larger air supply volume

  • No fouling/scaling

  • Economically favorable

  • Simplified maintenance

  • No contamination from process flow

Page 22

Disadvantages of Air Cooled Heat Exchanger

  • Cooling limit by ambient temperature

  • Increased electrical equipment

  • Higher initial costs

  • Leakage hazards

  • Sensitive to rain

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Components of Air Cooled Heat Exchanger

  • Heat transfer bundles

  • Air-moving device (fan or stack)

  • Driver for fan rotation

  • Support structure

  • Optional maintenance walkways

  • Louvers for temperature control

  • Variable pitch fan hub for savings

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Plate Heat Exchanger

  • Composed of multiple thin plates for large surface areas

  • Plates separated by rubber gaskets, arranged in troughs.

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Comparison of Heat Exchangers

  • Plate Type:

    • Higher surface area, low temperature and pressure use, suitable for non-contaminated fluids.

  • Tubular Type:

    • Designed for high-temp and pressure, lower surface area, can handle contaminated fluids.

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Problems in Heat Exchangers: Fouling

  • Definition: Formation of scale on heat transfer surfaces.

  • Effects:

    • Reduce heat transfer and flow

    • Decrease efficiency

    • Increase differential pressure

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Problems in Heat Exchangers: Tube Leaks

  • Effects:

    • Reduction in heat transfer

    • Contamination of products

    • Decreased efficiency

  • Solution: Tube replacement or plugging

Page 33

Problems in Heat Exchangers: Corrosion

  • Effects:

    • Reduced tube and shell thickness

    • Increased differential pressure

    • Decreased efficiency

  • Solution: Tube replacement or coating

Page 34

Monitoring in Heat Exchangers

  • Input/output temperature

  • Fluid velocity

  • Approach temperature

  • LMTD (log mean temperature difference)

  • Differential pressure

  • External leakages

  • Visual inspections for anomalies

Page 36

Heat Exchanger Analysis

  • Engineers need to:

    • Select heat exchangers for specified temperature changes

    • Predict output temperatures of fluids using two methods:

      • LMTD method

      • Effectiveness - NTU method

Page 37

Heat Transfer Principles

  • 1st Law of Thermodynamics:

    • Heat transfer rate from hot fluid equals that of the cold fluid.

    • Equations for heat transfer relating mass flow rates and specific heats.

Page 38

Phase Change Analysis

  • Nomenclature for evaporation and condensation processes.

  • Significance of the mean temperature difference.

Page 40

Log Mean Temperature Difference Method

  • Essential in heat exchanger sizing.

  • Emphasizes the importance of using LMTD over arithmetic mean in calculations.

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Counter-Flow Heat Exchangers

  • Outlet temperatures and temperature differentials.

  • Comparison with parallel-flow exchangers regarding area and efficiency.

Page 45

Stepwise LMTD Method for Heat Exchanger Selection

  • Select heat exchanger type

  • Determine unknown temperatures and heat transfer rates

  • Calculate LMTD and correction factor

  • Select heat transfer coefficient

  • Calculate required surface area.

Page 61

Example Problem Using Effectiveness NTU Method

  • Using a counter-flow heat exchanger for water heating with geothermal fluid.

  • Determination of heat capacity rates and maximum heat transfer.

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Solution Steps Using Effectiveness NTU Method

  • Identifying heat capacity rates

  • Computing maximum heat transfer

  • Establishing effectiveness and NTU relations

Page 65

Observations from Effectiveness Relations

  • Insights on effectiveness ranges and economic justifications in large NTU values.

    • Effectiveness trends favor counter-flow configurations.

Page 66

Heat Exchanger Selection Criteria

  • Discusses the relationships between design decisions and operational efficiencies.